The principal source of helium is its separation from natural gas streams prior to the natural gas streams being used as a fuel or as a feedstock. Natural gas streams can contain up to about 10 percent helium. It is economically feasible to recover helium from a natural gas stream down to a content of about 0.1 percent.
A conventional way to recover helium from natural gas is to use a cryogenic process. Water, carbon dioxide and any sulfide gases are first removed by scrubbing with monoethanolamine and diethylene glycol. The gas stream is then flowed through a low temperature heat exchanger and the heavy hydrocarbon fraction is separated and recovered. Free of heavy hydrocarbons, the gas stream is reduced in temperature to about 116.degree. K. This liquefies the remaining hydrocarbons which are removed. The resulting crude helium is then reduced in temperature to about 77.degree. K. to remove any remaining traces of hydrocarbons and to produce a helium stream, containing small amounts of nitrogen, argon, neon and hydrogen. The helium stream is pressurized to 17.3 MPa at 77.degree. K. and nitrogen and argon are separated. The remaining neon and hydrogen are removed by an activated carbon adsorbent.
Helium also can be separated from a helium and other gas containing stream by means of pressure swing adsorption. Such a process is disclosed in U.S. Pat. No. 5,089,048. This patent discloses a pressure swing adsorption system for helium enrichment. The process in this patent can be used with helium streams which contain less than 10 percent helium. The process consists of a three step pressure build-up phase, an adsorption phase, a three step pressure relief phase, and an evacuation phase. In the pressure build-up phase, a cocurrent first depressurization gas is flowed cocurrently into an adsorbent bed which has been evacuated to increase the gas pressure in this bed. This is followed by a countercurrent flow of a second countercurrent depressurization gas from another adsorbent bed which has completed an adsorption phase. This is then followed by a countercurrent flow of product gas to bring the bed up to the operating pressure. This process will produce a purified helium stream but at a lower efficiency. One problem is that there is a loss of product helium in the gases that are discharged as waste gases. Since the amounts of helium in the waste gas are relatively high, their loss creates an inefficiency in the process. In the processes of the present invention, helium is maintained in the pressure swing adsorption system as a gas inventory and not removed as part of a waste gas or off-gas. In addition the multi-step pressurization and depressurization techniques are not used.
European Patent 092,695 and U.S. Pat. No. 3,636,679 also disclose pressure swing adsorption systems for helium purification. In European Patent 092,695, the feed gas should contain about 50 to 95 percent by volume helium. It is not suitable for gas streams containing less than about 50 percent helium, and is clearly not useful where the helium content of the gas stream is less than about 25 percent helium.
These pressure swing adsorption processes do not economically produce a helium product having a purity of greater than 98 percent from a feed stream containing less than 10 percent helium.